Room: Exhibit Hall
Purpose: Electron pencil beam dose calculation algorithm is used for by many treatment planning systems (TPS) for electron therapy treatments. Recently, more TPS have implemented Monte Carlo (MC) dose calculation for electron therapy treatments. The purpose of this work was to retrospectively evaluate possible dosimetric differences between pencil beam and Monte Carlo calculations of previously treated patients.
Methods: Twenty previously treated patients with electrons were included in this study. The electron energies used for the treatment ranged from 6MeV to 15MeV. The treatment sites included sinuses, ears, testicles, skin, and intra-mammary nodes. All patient treatment plans were initially created using the Pinnacle TPS with the 3D Electron dose calculation algorithm. The plans were then recalculated using the Monaco TPS with the electron Monte Carlo. The dose grid resolution was kept to 3mm^3 for all calculations. For the Monaco calculations, the number of histories was set to 50,000/cm^2. The dose were compared using DVH, and differences within the isodoese lines.
Results: Differences between the two dose calculation algorithms were observed for the majority of the cases. The dose differences were within 1% for cases with minimal tissue inhomogeneities. Larger differences of more than 3% were observed where inhomogeneities were present (sinus, skin folds, skull). The DVH analysis showed higher doses for most of the organs when calculated with Monaco as well as broader and deeper dose distributions. In the cases of photon-electron beam matching, the dose on the photon side increased by 2-5%.
Conclusion: It is important to carefully consider the electron dose calculation accuracy when the treatment site has tissue inhomogeneities, as in the case of air-bone-tissue interfaces. When photon beams and electron beams are matched, consideration should be given to the hotspot in the photon side and if necessary measurements should be made to verify the dose.
Funding Support, Disclosures, and Conflict of Interest: Research is partially supported by ELEKTA, Inc.